1. Field of the Invention
The present invention relates to a supercharging pressure control device for an engine equipped with a mechanical supercharger and, in particular, to a supercharging pressure control device that controls supercharging pressure by means of a bypass valve provided in an air passage bypassing the supercharger.
2. Description of Related Art
In order to cause a mechanical supercharger, directly driven by an engine, to put out supercharging pressure, supercharging pressure control devices open and close a bypass valve, provided in a bypass passage communicating portions of an intake passage upstream and downstream from a supercharger disposed between a throttle valve and a surge tank, in accordance with engine load and engine speeds. Such a supercharging pressure control device is known from, for instance, Japanese Unexamined Patent Publication No.2 - 283816. The bypass valve is generally of a type equipped with a diaphragm-type negative pressure operated actuator or, otherwise, may be an electric throttle.
In the supercharging pressure control, by means of controlling opening of the bypass valve as explained above, a target supercharging pressure is established in accordance with engine speeds and engine load which is generally represented by throttle openings by looking up a data map of target supercharging pressure with respect to bypass valve opening. Then, negative actuating pressure is developed so as to open or close the bypass valve for putting out the target supercharging pressure. In this instance, the flow rate or volume of bypassing air in the bypass passage depends upon openings of the bypass valve and the pressure difference across the bypass valve, or the difference in pressure upstream and downstream of the supercharger and, consequently, changes with changes in pressure difference even for the same bypass valve opening. Consequently, the accuracy of traction control and/or the accuracy of torque reduction control in relation to automatic transmissions worsens. For instance, when the supercharging pressure has to be 100 mm Hg for an engine speed of 2,000 rpm and a 1/4 throttle opening but has to fall to the same pressure, i.e. 100 mm Hg, during traction control for a full throttle opening, the pressure downstream from the throttle valve, or upstream from the supercharger, varies greatly between during the normal supercharging pressure control with a 1/4 throttle opening and during the traction control with a full throttle opening in spite of the same target supercharging pressure of 100 mm Hg. Therefore, the accuracy of traction control or the like worsens if openings of the bypass valve are set unconditionally for target supercharging pressure on the premise of performing the normal supercharging pressure control. In order to solve this problem, in the conventional supercharging pressure control, it is essential to perform control with openings of the bypass valve which are set by looking up a three dimensional map of parameters such as engine speed and throttle valve opening as well as target supercharging pressure. However, this kind of control is practically difficult. In addition, when negative actuating pressure is controlled to cause the bypass valve to attain an opening established in accordance with a target supercharging pressure, it is necessary for the supercharging pressure control device to have a separate mechanism that corrects variances in operative characteristics, peculiar to the supercharger and the control device, from intended or standardized operative characteristics. Further, it is also necessary for the supercharging pressure control device to have a sensor that detects openings of the bypass valve and a sensor that detects flow rates at which air flows through the bypass valve. These additional structural elements cause an unavoidable increase in cost and complexity in the structure of the supercharging pressure control device.